1. Field of the Invention
The present invention relates to a voltage reference circuit and more particularly, to a voltage reference circuit that is able to generate two reference voltages each having a temperature dependence, which is operable at a low supply voltage, and has a simple circuit configuration.
2. Description of the Prior Art
To generate two reference voltages each having a fixed temperature coefficient, conventionally, two independent voltage reference circuits have been used in combination.
At present, there have been known a practical voltage reference circuit that can generate simultaneously two reference voltages each having a specific temperature characteristic.
A voltage reference circuit has been used for various electronic circuits to provide a specific biasing voltage therein. Especially, the "bandgap reference" circuit has been well known and popularly and widely used for the electronic circuits requiring high stability and high accuracy.
The "bandgap reference" circuit is realized by combining a first voltage dependent upon the base-to-emitter voltage V.sub.BE of a bipolar transistor having a negative temperature coefficient with a second voltage dependent upon the thermal voltage V.sub.T having a positive temperature coefficient, resulting in a wanted temperature dependence. A positive, negative or zero temperature coefficient can be generated by proper weighting the first and second voltages.
The base-to-emitter voltage of a bipolar transistor is approximately 600 mV, which has a negative temperature coefficient of approximately -2 mV/deg. Therefore, the output reference voltage of the "bandgap reference" circuit will have a zero temperature coefficient at approximately 1.205 V, which is equal to the bandgap voltage of silicon (Si). The reference voltage will be negative at a voltage lower than approximately 1.205 V, and positive at a voltage higher than approximately 1.205 V, which has been well known.
Recently, the integration scale of the Large-Scale Integrated circuits (LSIs) has been progressing more and more, and as a result, there has been an increasing possibility that a plurality of circuit blocks are integrated on the same semiconductor chip. In this case, the plurality of circuit blocks do not always require their bias voltages to have the same temperature dependence or characteristic. It has been often needed that the bias voltages of the plurality of circuit blocks are different in temperature dependence or characteristic.
To cope with the need of two reference voltages having different temperature coefficients, two separate voltage reference circuits may be simply used in combination. However, the combination of the two separate voltage reference circuits will cause some problems relating to circuit scale and current consumption.